EBay, Collectors, Flea Market & Antiques

Research each item and estimate cost plus sales tax. Any other expenses also count. Just inventory everything and make list on paper or a spreadsheet. Actually you just need to estimate year and know where to find digital catalogs like Sears, JC Pennies, Montgomery Ward use any of those after you sell something. It will only matter if you break $600 somewhere. You can put the stuff on a Schedule C, where you can take business expenses off the top before and normal income deductions.
Not accounting advice, just ideas to direct your research. Use DuckDuckGo to look for current advice about selling personal items bought years ago. If your reselling stuff it will be tougher.